Fairy in endless danger (4)
I believe the answer is:
peri
'fairy' is the definition.
(Persian mythological being)
'endless danger' is the wordplay.
'endless' means to remove the last letter.
'danger' becomes 'peril' (peril is a kind of danger).
'peril' with its last letter taken off is 'PERI'.
'in' is the link.
(Other definitions for peri that I've seen before include "Beautiful fairy" , "Fairy queen" , "Fairy could be ripe" , "Fairy of Persian myth" , "Persian fairy or genie" .)